There is a strong need for substrates with lattice parameters typically unattainable thorough the use of commercial substrates. The common elemental (Ge or Si) and binary III-V semiconductors (GaAs, InP) represent a few fixed lattice constants. A strategy to expand the range of substrate lattice parameters involves the use of metamorphic buffer layer (MBL) structures. This strategy begins with a binary substrate, e.g. GaAs, and grades the composition and hence lattice parameter through the heteroepitaxial growth of a semiconductor alloy, e.g. InxGa1-xAs, starting at the binary composition. A layer of constant composition is grown at the top of the graded layer possessing the lattice constant required by the device application. This strategy is attractive due to the availability of high-quality, large area GaAs substrates and ease of controlling the composition of the MBL structure during growth via the variation of a single parameter (e.g. ratio of In/Ga) to achieve the desired in plane lattice constant.
However, growth of MBL structures containing high levels of strain relaxation and a low threading dislocation (TD) density is challenging. Often both goals are difficult to achieve in tandem, as relaxation of the MBL structure requires the introduction of misfit dislocations (MDs) to relieve strain. The introduction of these MDs is usually concurrent with the introduction of threading dislocations (TDs) which nucleate at the growth surface as a dislocation loop which propagates to the heterointerface, leaving a threading segment through the thickness of the MBL structure. In the InxGa1-xAs and InxAl1-xAs systems for example, a number of MBL structures such as constant composition, step graded, and linearly and non-linearly graded MBL structures have been grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) with varying degrees of success in mitigating TD density. Typically, a significant residual strain remains in the completed MBL structures.